The present invention relates to an extrusion head for foamed material, in particular for polyvinylchloride-based foamed material.
Extrusion heads for formed material, for example for PVC with the addition of substances which causes its foaming, are known which substantially comprise a head body with elongated configuration in which a longitudinal cavity is defined; an element known as a "male" element is accommodated in said longitudinal cavity. Said male element defines, in co-operation with the internal walls of the cavity in which it is contained, a passage for the material which is pushed, with a present pressure, along said passage in the semi-liquid or pasty state.
The passage for the material leads outward at a longitudinal end of the head through an extrusion hole which is shaped correspondingly to the configuration of the extruded element to be obtained.
During extrusion, the material foams and the extruded element assumes its final configuration, which is stabilized during cooling by making said extruded element pass through "gauges" which are substantially constituted by cooled blocks in which a through cavity is defined which reproduces the profile of the extruded element to be obtained. In some cases this cavity is kept at negative pressure so as to improve the outer profile of the extruded element, achieving a high degree of finish.
Said known types of extrusion head allow to obtain extruded elements with a hollow cross section with good results, but have some disadvantages in the execution of solid cross-section extruded elements.
Great difficulty in execution is in fact encountered if an extruded element with solid cross is to be obtained with know extrusion heads, for example if the extruded element must have good mechanical resistance substantially through its entire thickness.
It is indeed very difficult to obtain a solid cross-section extruded element by increasing the thickness of the walls of said extruded element at its exit from the extrusion hole, i.e. by reducing the dimensions of the end of the male element which faces said hole; on the contrary, a cavity is very often always present at the center of the extruded elements. This effect derives from the fact that the material tends to foam more easily toward the outer surface of the extruded element and with great difficulty toward its center.
Even when the outer surface of the extruded element has set, the internal layers of the material tend in any case to foam from the inside outward and not vice versa.
This behavior of the material furthermore produces a high density of the extruded element proximate to its outer surface and a reduced density proximate to its center.
In order to obtain a solid cross-section extruded element with conventional extrusion heads it is necessary to considerably increase the thickness of the extruded element exiting from the extrusion head, with the disadvantages of using a considerable amount of material, most of which, at the end of extrusion, is practically not formed proximate to the outer surface of the extruded element.